Im a little weary of a FF buff because of the superiour tracking skill in hammer... Plus tulips are pretty great if a bit expensive. That said i think your target of 35 fuel and 12 pop seems reasonable to me
the tracking last ten seconds, and the Firefly is slower than most tanks to begin with.
You can get a second shot if you're lucky.
beside. I'm not asking for a reload buff. 200 damage with 8s reload is fine.
440 mp 155 fuel 14 pop is borderline extortion for the FF's performance. That isn't including the tulip upgrade cost.
You mention nothing about how its midrange modifier is well above the standard TD, which gives it insane accuracy. 12 popcap is just a big no. The unit could use a SMALL tweak in the cost department. There is no need to COMPLETELY rework the entire unit when small changes could easily fix some of its outlying issues.
the Firefly's mid range advantage is the mid range distance, not modifier, and in effect it only affect accuracy and Penetration.
secondly, Firefly isn't really more accurate in practice.
both the firefly and jp4 get .04/.05/.06 accuracy scaling. Against a 22 size target that's already a 88% hit rate at 60m. The FF will hit 100% accuracy faster, but this is the kettle calling the pot black.
If you would like to know more about TD accuracy, I made a more in-depth topic earlier:
https://www.coh2.org/topic/69042/tank-destroyers-should-be-less-accurate